A non-servo drum brake provides a support assembly on a backing plate to pivotally anchor a pair of brake shoes. A hydraulic actuator is engageable with the pair of brake shoes to pivot the brake shoes about the support assembly from a rest position to a braking position. An adjustable strut extends between the pair of brake shoes to compensate for lining wear and a parking arm cooperates with the strut to move the pair of brake shoes to the braking position during a parking brake application.
The non-servo drum brake is frequently used as a rear brake on vehicles with front wheel disc brakes. The non-servo rear drum brake is sufficient to assist the front wheel disc brakes in stopping the vehicle. It is designed to develop less torque than the disc brake to reduce the chance of wheel skidding. A problem exists during a parking brake mode because the rear non-servo drum brake is required to hold the vehicle stationary without any assistance from the front wheel disc brake. One solution to this problem is to free the support assembly during a parking brake mode as taught by U.S. Pat. application Ser. No. 12,991, filed on Feb. 21, 1979. Such an arrangement converts the non-servo drum brake to a Duo-Servo drum brake only during a parking brake application.
A duo-mode drum brake is disclosed in U.S. Pat. application Ser. No. 80,809, filed on Feb. 21, 1979, wherein a non-servo drum brake is converted to Duo-Servo operation during a parking brake application. The brake is converted to Duo-Servo mode by a mechanism which converts the support assembly for the brake shoes so that brake torque generated by one brake shoe is transferable through the support assembly to the other brake shoe, rather than being transferred to the backing plate. Consequently, the other brake shoe is urged into tighter engagement with the brake drum. The mechanism has only a limited movement between the position in which the support assembly defines a non-servo brake and the position in which the assembly defines a Duo-Servo brake. The mechanism is connected to the parking arm by a link and a connector such that movement of the parking arm, or in the alternative, movement of the mechanism, in response to parking cable movement is determined by the balance of friction and spring forces on the parking lever and on the mechanism, respectively. Tests of drum brakes constructed according to the invention of the application Ser. No. 80,809 have shown that part of the limited motion of the mechanism may be used prior to the time that the brake shoes are moved into full engagement with the brake drum. Consequently, the brake shoes may not be applied to the brake drum in a manner to most efficiently provide brake torque for parking purposes.